1. Field of the Invention
This invention relates to an LCD device, and more particularly, to an LCD device of the reflective type or transflective type of the lateral-electric-field drive mode.
2. Description of the Related Art
Conventionally, there have been suggested LCD devices of reflective type or transflective type in which a pixel electrode and a common electrode are formed on the same substrate, as in an IPS mode or FFS mode, and LC molecules are switched using a lateral electric field generated between the pixel electrode and the common electrode to thereby display an image on the screen. For example, in Patent Publications JP-2003-344837A1, JP-2005-338256A1, there have been described transflective type lateral-electric-field drive modes using a reflection mode as a normally black mode under which absence of applied voltage on the LC layer represents a dart state or black. Furthermore, in JP-2006-180200A, there has been described a transflective type lateral-electric-field drive mode using a reflection mode as a normally white mode under which absence of applied voltage on the LC layer represents a bright state or white.
Hereinafter, a transflective type LCD device of the lateral-electric-field drive mode described in JP-2003-344837A1 will be described. FIG. 25 shows a schematic sectional view indicative of an LCD device described in this publication. The LCD device 200 includes a pair of substrates including TFT substrate 214 and counter substrate 212, which oppose each other, an LC layer 213 which is sandwiched between the TFT substrate 214 and the counter substrate 212, and polarizing films 211, 215 which are attached to the external surfaces of the TFT substrate 214 and counter substrate 212, respectively, far from the LC layer 213. Between the LC layer 213 and the polarizing film 215, a half-wave (λ/2) plate 218 is inserted. In FIG. 25, the LCD device 200 has a backlight unit (not shown) that irradiates backlight to the LC layer 213 through the polarizing film 215. Furthermore, on the surface of the TFT substrate 214 and counter substrate 212 near the LC layer 213, a horizontal orientation film (not shown) is formed. The angle formed between the orientation directions of the two horizontal orientation films represents a twist angle of the LC layer 213.
The LCD device 200 includes in each pixel a transmissive area 222 that allows the light incident from the backlight unit to transmit from the polarizing film 215 toward the polarizing film 211, to thereby display an image on the screen, and a reflective area 221 that reflects the light incident from the outside through the polarizing film 211 and reflected by a reflection film 216, to thereby display an image on the screen. A first insulating film 217 is formed on the surface of the TFT substrate 214 near the LC layer 213. In the reflective area 221, a second insulating film 242 is formed on the first insulating film 217, and the reflection film 216 is formed thereon. On the reflection film 216, a third insulating film 241 is formed, and on the third insulating film 241, lateral-electric-field drive electrodes including pixel electrode 235 and common electrode 237 are formed. On the other hand, in the transmissive area 222, lateral-electric-field drive electrodes including pixel electrode 236 and common electrode 238 are formed on the first insulating film 217 arranged on the TFT substrate 214.
In the reflective area 221, the pixel electrode 235 and common electrode 237 extend parallel to each other, and the LC layer 213 is driven by a lateral electric field generated between the pixel electrode 235 and the common electrode 237. In the transmissive area 222 either, the pixel electrode 236 and common electrode 238 extend parallel to each other, and the LC layer 213 is driven by a lateral electric field generated between the pixel electrode 236 and the common electrode 238. The second insulating film 242 and third insulating film 241 adjust the difference between the LC cell gap of the reflective area 221 and the LC cell gap of the transmissive area 222. Specifically, when the gap of the LC layer 213 in the transmissive area 222 is set to ½ wavelength (λ/2) of light, the gap of the LC layer 213 in the reflective area 221 is adjusted to ¼ wavelength.
FIG. 26A shows the polarizing axis direction of the polarizing film 211 and the LC orientation direction in the LC layer 213 of the above-described LCD device, and FIG. 26B shows the polarized state of light in the reflective area 221. It is defined, as shown in FIG. 26A, that the polarizing axis of the polarizing film 211 and the LC orientation direction in the LC layer 213 is at 90 degrees. In this notation, as shown in FIG. 26A, upon absence of applied voltage on the LC layer, a 90-degree-linearly polarized light passed by the polarizing film 211 directly passes through the LC layer 213, and is reflected by the reflection film 216 with its polarized state being kept linearly polarized. The reflected linearly polarized light directly passes through the LC layer 213, and passes by the polarizing film 211, whereby the image on the screen assumes a bright state or white. Upon presence of applied voltage on the LC layer, the oriented angle of the LC layer 213 assumes 45 degrees, and the linearly polarized light passed by the polarizing film 211 passes through the LC layer 213 to assume clockwise-circularly polarized light, which is reflected by the reflection film 216 to assume counterclockwise-circularly polarized light to pass through the LC layer 213, and advances toward the polarizing film 211, as a 0-degree-linearly polarized light. Accordingly, the light is blocked by the polarizing film 211, whereby the image on the screen represents a dark state or white, resulting in a normally white mode.
FIG. 27A shows another example of the polarizing axis direction of the polarizing film 211 and the LC orientation direction in the LC layer 213 of the above-described LCD device, and FIG. 27B shows the polarized state of light in the reflective area 221. As shown in FIG. 27A, a case is considered in which the polarizing axis of the polarizing film 211 is set to 90 degrees, and the LC orientation direction in the LC layer 213 is set to 45 degrees. In this case, upon absence of applied voltage, the 90-degree-linearly polarized light passed by the polarizing film 211 passes through the LC layer 213 to assume a clockwise-circularly polarized light, which is reflected by the reflection film 216 to assume a counterclockwise-circularly polarized light. Since the counterclockwise-circularly polarized light passes through the LC layer 213 to assume a 0-degree-linearly polarized light, the light is blocked by the polarizing film 211, thereby representing a dark state or black. Upon presence of applied voltage, the oriented angle of the LC layer 213 assumes 0 degree, and the 90-degree-linearly polarized light passed by the polarizing film 211 passes through the LC layer 213 to be reflected by the reflection film 216 with the polarized angle being kept at 90 degrees. The reflected light from the reflection film 216 passes through the LC layer 213 with its polarized state being kept 90-degree-linearly polarized, and is emitted from the polarizing film 211, whereby the image on the screen represents a bright state or black, representing a normally black mode.
FIG. 28 shows the oriented state of LC molecules in the LCD device shown in FIG. 25 upon presence of applied voltage. Upon presence of applied voltage, a lateral electric field is generated between comb teeth electrodes, or between pixel electrode 235 and common electrode 237, and LC molecules in the LC layer 213 are oriented along the direction of the lateral electric field. However, since the lateral electric field is not applied to the LC layer 213 on the comb teeth electrodes, the LC molecules do not rotate thereon. More specifically, if the LCD device 200 is used as the normally white mode, the LC molecules on the comb teeth electrodes 235, 237 do not rotate. Therefore, the image on the electrodes 235, 237 stays “white” even when a voltage is applied to the LC layer, and the image assumes “black” only on the gap between the electrodes 235, 237, which raises a problem of lowering of the contrast ratio.
Specifically, when the configuration of a convex-concave surface of the reflection film 216 was uniform between the area overlapping the gap of electrodes and in the area overlapping the electrodes, even upon presence of applied voltage, the image on the electrodes stays bright, and the contrast ratio was 3:1 or lower at the maximum. With respect to the normally black mode, the LC molecules on the electrodes do not rotate similarly. Therefore, the image on the electrodes stayed “black” even when a voltage is applied to the LC layer, and the image assumed “white” only in the area overlapping the gap between the electrodes, which raises a problem of lowering the reflectance.